Systems, methods, and devices are provided for estimating when preventive maintenance of power capacitors is called for. Such a system may include, for example, a voltage sensor, a current sensor, and data processing circuitry. The voltage sensor may measure a voltage difference across a phase of a power capacitor. The current sensor may measure a current across the phase of the power capacitor. The data processing circuitry to determine a first instantaneous indication of a difference between a nominal capacitance of the power capacitor and an actual value of the power capacitor based at least in part on the measured first voltage difference and first current.